The present invention relates to a MOS-transistor bridge circuit.
As is known, MOS-transistor bridge circuits comprise four transistors, each connected in parallel to a respective flyback diode and controlled at the gate terminal by a circuit intended to ensure switching of the transistors according to three input signals, to obtain the desired operation, and powering a load at the desired current and/or voltage levels.
A known circuit of this type is shown in FIG. 1. Herein the MOS transistors 1-4, with the respective diodes 5-8, are driven by AND gates 11-14 according to the signals present on the inputs IN1, IN2 and EN. Respective inverter elements 15 and 16 connect the switching inputs of the AND gates driving the transistors of the same branch, so as to prevent their simultaneous switching on. On the transverse arm of the bridge an inductive load 9 is provided, and the bridge is connected between a supply voltage V.sub.CC and the ground through a resistor 10.
For the operation of the circuit of FIG. 1, suppose that each individual MOS transistor is in the on state when it receives a high signal at its gate terminal and, vice versa, is off if it receives a low signal. Accordingly, for each branch, the upper transistor is on when both the enable signal EN and the control or switching input IN1, IN2 are high. Vice versa the lower transistor of each branch is on when the control or switching signal is in the low state, but the enable signal is always high. Moreover the enable signal, if in the low state, is capable of switching off all four transistors simultaneously.
FIG. 1 also shows the diodes 5-8 connected in parallel to each transistor. These diodes may be formed for example by the intrinsic diodes obtained in the manufacture of the power MOS transistors (DMOS).
Accordingly, with a high signal on the gate terminal of each MOS transistor, the same is on and behaves like a resistor with value equal to R.sub.DS(ON) of the transistor with in parallel the intrinsic drain-source diode which intervenes in the inverse operating region to limit the excursion of the voltage across it. Vice versa, when the MOS transistor is off with V.sub.GS =0, the equivalent circuit is constituted only by the diode, which can thus conduct the current in a direction opposite to that of normal operation of the MOS transistor.
FIG. 2 illustrates the behaviour of the current I.sub.DS versus the voltage V.sub.DS, wherein the curve I, which is a straight line with slope R.sub.DS (ON), plots the resistive behaviour of the MOS transistor in on condition, while the curve II illustrates the behaviour, typical of the diode, when the transistor is off. From the plot of FIG. 2 it is furthermore apparent that with a negative current I.sub.1 during flyback with the MOS transistor off, an operating voltage V.sub.2 is obtained, entailing therefore a power dissipation equal to P.sub.diss1 =V.sub.2.I.sub.1.
Bridge circuits of the described type are extensively used for driving loads, in particular inductive loads such as motors and the like; however, due to the illustrated behaviour, in some cases they have an excessive power dissipation which it is desired to reduce.